1. Field of the Invention
The present invention relates to an injection-molding machine, and more particularly, to a method and an apparatus for controlling injection of an injection-molding machine, in which the injection of the injection-molding machine is controlled by means of a numerical control unit.
2. Description of the Related Art
Injection control in an injection-molding machine may be effected by two control methods, as shown in FIGS. 4 and 5. Namely, in a system of the type shown in FIG. 4, speed control is performed from an injection starr position, and, when a screw reaches a switching position for hold, the speed control is changed over to hold pressure control. Thereupon, hold is performed for a predetermined period of time, and a metering block in the next stage is entered. According to the system of FIG. 5, injection control is performed for a predetermined time after the start of injection, and, when the predetermined time for the injection control elapses, the injection control is changed over to the hold pressure control. When the hold time terminates, the metering block in the next stage is entered.
Conventionally, in controlling an injection-molding machine by means of a numerical control unit (hereinafter referred to as NC unit), the system of FIG. 4, not the system of FIG. 5, has been used. However, if the screw cannot inject resin at a predetermined injection speed and cannot, therefore, reach the switching position for hold within a normal required injection time, on account of the viscosity of the resin being too high or any other reason, the time required for the execution of an injection cycle is extended, thus lowering the efficiency of production. Also, the screw cannot follow up an output command, for use as a speed command, so that an error of a command for movement, compared with an actual screw position, is stored in an error register in the NC unit. If the value of the error reaches a predetermined level or higher, the NC unit stops pulse distribution, thereby possibly stopping the whole injection-molding machine in an unexpected manner.